Ultrasonic imaging has been extensively applied in virtually every medical specialty in the form of pulse echo B-mode tomography (See Wells, 1977). B-mode tomography or B-scan systems display echoes returning to an ultrasonic transducer as brightness levels proportional to echo amplitude. The display of brightness levels results in cross sectional images of the object in the plane perpendicular to the transducer.
With present ultrasonic transducer arrays, transmit focusing and receive mode dynamic focusing are achieved by proper timing of the transmit signals and delaying receive mode echoes. For example, a sectored linear phased array consists of a single group of transducer elements which is not only focused but also steered over a sector angle in transmit and receive (Tx and Rx) by properly timing the transmit signals and receive mode echoes (see yon Ramm et al., 1983).
In previous phased array imaging systems, the timing or phasing data is determined by assuming propagation of ultrasound pulses through a homogeneous tissue medium with a uniform velocity of sound, usually 1540 m/sec. The assumption of a constant velocity of sound in the body is also the design basis in all ultrasound scanning systems for converting round trip pulse-echo time of flight into target range in the image. Unfortunately, this simplest model of all human tissues is not valid. The body is actually composed of inhomogeneous layers of differing tissues (fat, muscle and bone) with bumps and ridges of varying thicknesses and different acoustic velocities. These layers intervene between the transducer and the internal organ of interest. The propagation velocity of ultrasound varies from approximately 1470 m/sec in fat to greater than 1600 m/sec in muscle and nervous tissue to as much as 3700 m/sec in bone (see Goss et al.). If an incorrect average velocity is chosen, B-scan imaging is known to result in an image range error and compound scan registration errors for all ultrasound systems.
Under the assumption of a uniform tissue medium of constant velocity, the presence of inhomogeneous tissues can also result in image artifacts, range shifts, geometric distortions, broadening of the transducer beam pattern which degrades the ideal diffraction limited lateral resolution, and increased side lobes which reduce the signal to noise ratio in the image. These problems occur in all types of pulse echo ultrasound systems to some degree.
The adverse effects of inhomogeneous nonuniform tissue layers have been analyzed by several investigators primarily in terms of unknown phase aberrations associated with the inhomogeneities introduced across the transducer aperture (see Trahey et al., 1991). Attempts have been made to overcome these aberrations using various signal processing techniques. These attempts include the B-scan phase correction techniques first described in Phillips et al. (Acoustical Holography, 1975), and later described in Smith et al. (NBS Pub. #525, 1978) and U.S. Pat. No. 4,817,614.
The phase aberration compensation method of Smith et al. in U.S. Pat. No. 4,852,577 is based on the understanding of medical ultrasound images of tissue which consists primarily of a random speckle interference pattern resulting from the phasor summation of echoes from a large number of fine scatterers within the transducer resolution cell (see Burckhardt, 1978; and Wagner et al., 1983). The echoes from these particles exhibit phases uniformly distributed over 0 to 2.pi. radians. Although the image brightness of an individual speckle is a random process, the mean image brightness and variance over an area is predictable.
Smith et al. in U.S. Pat. No. 4,852,577 have demonstrated that individual speckle spots change unpredictably from bright points to null as the phase function or aberration changes across the transducer aperture. However, the inventors have also demonstrated that the average image brightness of speckle in a region of interest is predictably decreased by transducer phase aberrations (see Trahey and Smith, 1988). Thus, an individual speckle in the image cannot be used as an image sharpening target. However, the average brightness of many speckles over a region of interest can be used as a quality factor in an image sharpening process for a phased array ultrasound scanner such as that disclosed in U.S. Pat. No. 4,852,577 issued to Smith et al. the disclosure of which is incorporated herein by reference as if set forth fully. In U.S. Pat. No. 4,852,577, the average brightness of the coherent sum of all of the elements in a phased array transducer for an entire region of interest in an image was utilized to correct for phase aberration. This method suffers from using poorly correlated data and an unstable reference phase. In contrast, the present invention utilizes a selected subset of the elements of the transducer array for the region of interest which was unexpectedly found to provide superior correction.
Another method for phase aberration correction has been described by M. O'Donnell et al. in U.S. Pat. Nos. 4,835,689 and 4,989,143 and by Hassler et al. in U.S. Pat. No. 4,817,614 the disclosures of which are incorporated herein by reference as if set forth fully. In this method for a region of interest in an ultrasound speckle image, a cross correlation function is calculated between two transducer array elements N and N+1 of a phased array system. The phased array scan data between these two elements is varied until a maximum is achieved in the cross-correlation function. The process is then continued with element N+2 versus N+1. This method of using cross-correlation relies on a product, i.e., multiplication, rather than an integral or a sum, as in the average speckle brightness technique, and is only performed between signals received at adjacent elements, not selected combinations of transducer array elements as in the instant invention.
As described above, O'Donnell et al. and Hassler et al. describe a phase aberration correction method that extracts a compensating phase at each element using a cross-correlation between it and its adjacent neighbor. They make no attempt to include data from more than the two elements involved when extracting this phase information, and their method is sensitive to noise, dead elements and/or an unstable reference phase. Additionally, the instant invention incorporates data from elements that have been previously corrected into the correction of the data for each element, which produces a more stable reference phase than the methods of O'Donnell et al. and Rachlin, described below.
A modification of the method of O'Donnell et al. has been proposed by D. Rachlin (1990). In this modification, a matrix of cross-correlation functions between the data from every transducer array element in the system is used to maximize the phase closure when estimating the aberrating function. This algorithm is currently too computationally intensive to be considered for high speed applications and does not incorporate selected transducer array elements or make use of previously corrected data as in the instant invention. By using data from all of the elements instead of only selected elements, this technique uses some poorly correlated data when extracting the compensating phase at each element and losses accuracy in determining the exact shift. It also does not attempt to incorporate data from previously corrected elements into its correction which could lead to an unstable reference phase.
Another proposed method to remove the effects of spatially distributed velocity inhomogeneities is the "time-reversal-mirror" technique (see M. Fink et al., 1989). In this method, the received signals are digitized and stored at every transducer array element and then retransmitted using a last-in, first-out scheme. This method does not directly extract a compensating phase aberration profile and still has considerable technical difficulties to overcome before it can be applied to speckle targets.
The present inventors have recently determined the key features for successful phase aberration correction (see Trahey and Freiburger, 1991). For a phase aberration correction system to work optimally it needs to be insensitive to noise, insensitive to dead, missing or blocked elements, implemented on two dimensional arrays, use highly correlated data, and use a stable reference phase while correcting each of the elements. If a phase aberration correction method is sensitive to noise or to dead elements it will perform poorly at extracting the compensating profile and may even introduce more severe phase aberration than that which is being removed. Removing the effects of phase aberration in only one dimension does little or nothing to improve image quality because there are still components of the aberrator in the other dimension. Phase aberration correction performance can be greatly enhanced by aligning the phase of element groups which have highly correlated data because the exact phase relationship between signals becomes obscured when the data is uncorrelated. Lastly, if the reference phase changes when correcting the data at different elements in an array then the change in reference phase will still be present after the correction over the entire array is complete. This left-over difference in phase between elements is still phase aberration and degrades image quality. Previous phased array ultrasonic imaging systems did not fully utilize these features and perform less than optimally at compensating phase aberration.
In view of the above discussion, it is, therefore, an object of the present invention to provide an on-line adaptive ultrasonic pulse echo phased array imaging device which corrects for transducer phase aberrations and optimizes spatial resolution.
It is a further object of the present invention to provide an adaptive pulse echo phased array imaging system which is insensitive to noise.
Yet another object of the present invention is to provide an adaptive pulse echo phased array imaging system which is insensitive to dead, missing or blocked elements of the ultrasonic transducer array.
It is another object of the present invention to provide an adaptive pulse echo phased array imaging system which uses highly correlated data and a stable reference phase to correct phase aberrations in each element of the ultrasonic transducer array.
It is still a further object of the present invention to provide an adaptive phased array imaging system that can use signals received on selected subsets of the receive array. These subsets can include signals received within a radius of a given element or signals with previously corrected phases received within a radius of a given element or signals from previously corrected elements.
Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings.